Fasteners in the form of bolts and cap screws are usually produced using automatic bolt making cold forming machines. In these machines, wire is typically fed into the machines with the wire being of the diameter larger than the shank diameter of the bolt being made. This usually involves drawing the wire through a die which reduces its diameter to within about one hundredth of an inch larger than the final shank diameter. The rod is cut to length to form a slug which is then formed into a bolt blank prior to threading. The bolt blank has a first portion which that is upset at the end of the rod to form a head, a second unthreaded shank portion and a reduced diameter third portion that is subsequently formed into a thread. The third portion of the bolt blank is typically forward extruded into the cavity die in the bolt making machine to produce a diameter close to that of the pitch diameter of the subsequently formed thread. The blank is then ejected from or kicked out of the bolt making machine. Thereafter, a thread is usually formed on the third portion of the bolt blank in a thread rolling machine in which dies deform the blank to form the helix thereon having a thread crest diameter slightly less than the nominal diameter of the bolt. In order for the bolt to center in a cylindrical bolt hole in an element to be assembled by the bolt, it is necessary that the shank portion of the bolt have a diameter only slightly smaller than the diameter of the cylindrical bolt hole.
There are advantages to reconfiguring the shank portion of the bolt, which advantages are nullified if a bolt manufacturer must substantially redesign very expensive bolt making machines or reduce the cross sectional area of the bolt shank by subsequent manufacturing steps. To date the advantage of reducing the cross section of the shank portion has not been commercially exploited by reducing the shank cross sectional area of bolt blanks produced in cold forming bolt making machines. Prior art reduced area shank bolts are of designs which could not be made in their final form in conventional bolt making machines because their blanks could not be ejected from single piece cavity dies. The shape of prior art reduced area bolt shanks would interfere with ejecting the bolt blanks from single piece bolt blank cavity dies. Consequently, the advantages of such bolt designs are not generally available except as special purpose bolts which are relatively expensive and are usually custom manufactured for specific machines and assemblies.
Among the advantages of reducing the diameter of the shank portion are savings in steel consumption. Due to rapidly growth and manufacturing throughout the world, steel and other metals are becoming increasingly expensive. Use of the present invention results in about 10-15% reduction in the quantity of metal needed to produce clamping bolts for specified uses and results in material savings and price stabilization not only for bolt manufacturers but for their customers. By reducing the amount of metal required to manufacture bolts, weight reduction can be achieved in vehicles which use bolts, such as but not limited to, engines and airframes for aircraft and similarly for automotive vehicles which result in substantial fuel savings. For example, there is a cost of about five-hundred dollars per year for an airline to fly each pound of weight. A reduction in the weight of each of the many steel bolts used in aircraft structures, such as landing gear, airframes and engines, therefore results in substantial savings to air carriers.
The mechanical advantages of having shank portions with reduced cross section include providing additional elastic stretch and reduced stress throughout threaded fasteners upon application of external joint parting loads, as compared to conventional fasteners with shanks of nominal diameter. These advantages become available with fasteners such as bolts and cap screws which can be configured so as to be ejected from conventional bolt making machines.
Prior art patents describe ways to manufacture reduced area shank threaded fasteners by employing additional manufacturing steps to reduce the shank cross sectional areas of fasteners manufactured with conventional nominal bolt diameter round shanks. Prior art patents typically show reduced cross sectional area fasteners produced by reducing the shank diameter below the equivalent diameter of the thread stress area or by forming windows in the shanks or by reduced diameter sections. These prior art fasteners with reduced area shank cross sections cannot be manufactured in conventional automatic bolt makers because they cannot be ejected from cavity dies due to recessed windows or round cross sections in the shank with closed ends.